Reading FAT table

Hi,
I'm doing a program which reads the FAT table of a partition and calculates
the amount of free space on that partition.
I read the FAT table of a 1,5Gb partition. The partition is 80% free but I get
with the program that the partition has 5% free space.
I don't know what's wrong interpretating the FAT.

unsigned long long infoSectorOffset; /* FAT32 only */
unsigned long long fatOffset;
unsigned long long rootDirOffset; /* non-FAT32 */
unsigned long long clusterOffset;
unsigned long long bootSectorBackupOffset;

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